Safety lock for electric plugs



April 24, 1956 A. PERRAULT ETAL 2,743,425

SAFETY LOCK FOR ELECTRIC PLUGS Filed Nov. 9, 1950 4&

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E .L I; 74% V fzzz/emfalj 9 i X3355; 9 WW /6 j vfial/%%wy l r /J/ i United States Patent 2,743,425 I SAFETY LOCK FOR ELECTRIC PLUGS Art Perrault and Charles V. Harvey, Forest Lake, Minn. Application November 9, 1950, Serial No. 194,846

1 Claim. 01. 339-90 a poor contact and that the poor contact is frequently caused by the male plug being partially withdrawn from the receptacle; This partial withdrawal is often the result of someone tripping on the electric cord leading from the device to the male plug, or attempted moving of the device to a distance from the receptacle greater than the length of said cord, or the cord being pulled on by a child, household pet, or in some other manner. Such a poor connection causes the supply of current reaching the device to have substantial damaging results to the more sensitive parts thereof.

In addition to the above the conventional plugs, currently in use today, can be readily removed by infants and small children. Thereafter the receptacle is accessible to them for the insertion of metal objects such as hair pins and the like with consequent extreme danger to the infant. Our invention is directed toward eliminating these hazards.

It is a general object of our invention to provide a new and improved electric safety plug of cheap and simple construction and of high efficiency.

A more specific object is to provide an electric safety plug which will positively prevent itself from being accidentally partially withdrawn from the receptacle and thereby causing large variations in the amount of current passing therebetween with consequent danger to the electrical device connected with the safety plug.

Another object is to provide an electric safety plug having inherent locking means therein for positively locking the male plug to the receptacle after its insertion therein to preclude the two from becoming separated unintentionally. 7

Another object is to providean electric safety'plug which can be readily used in conjunction with a standard receptacle and without requiring installation of a special type receptacle.

A further. object is to provide an electric safety plug which will prevent an infant from removing the same from the receptacle and thereby gaining access to the interior of the latter with consequent danger.

These and other objects and advantages of our invention will more fully appear from the following description made in connection with the accompanying drawings, wherein like reference characters refer to similar parts throughout the several views, and in which:

Fig. l is a front elevational view of one embodiment of our invention inserted in a double receptacle of the standard type and locked thereto;

Patented Apr. 24, 1956 ing a relatively flat forward face surface 7 and a restricted rear end portion 8. Formed on the rear end of the rear end portion '8 is an outwardly extending ridge 9. Mount ed within the main body 6 of insulating material and extending outwardly therefrom through its forward face 7 is a pair of electrical conductor prongs 10. These prongs 10 are adapted in the conventional manner to be secured to the respective wires 11 of an electrical cord 12. As shown, the main body 6 of insulating material is adapted to receive, from the rear in its central portion, this electrical cord 12 and to permit the connection of the wires 11 to the respective prongs 10.

A shell member indicated generally as 13 is mounted.

in encircling relationship 'on the main insulating body 6 for free rotation thereon. This shell 13 has an open face 14 and an inwardly extending flange15 adjacent its rear end. The size of the flange 15 is such as to leave an aperture 16 in the rear end of the shell 13, the diameter of this aperture 16 being slightly smaller than the diameter of the ridge 9 on the main body 6 of insulating material.

Formed in the forward end portion of the shell 13) is a longitudinally extending slot 17 which communicates with the face 14 and extends rearwardly a slight distance. -A circumferentially arranged slot 18 is also formed in the forward end portion of the shell 13 and is arranged at right angles to the direction of extension of the slot 17 and communicates with'the innermost portion thereof. To assemble the main body 6 and the shell 13, the shell 13 is heated slightly and the resulting expansion of the same is sufiicient to permit the ridge 9 to pass through the aperture 16 defined by the inwardly extending fiange 15. The subsequent cooling of the shell 13 will cause the diameter of the aperture 16 to again be restricted to a point where the main body 6 will thereafter be secured to the, shell 13 to prevent appreciable relative longitudinal movement therebetween. The flange 15 will there after ride on the reduced portion 8 of the main body 6 and the shell 13 will be free to rotate with respect to the main body.

To adapt. any standard receptacle for use of our invention it is only necessary to replace the conventional screw, which holds the wall plate 19 to the wall, with a double headed screw indicated as 20. This double headed screw 20 is screwed inwardly until the inner head 21 rests against the plate to retain the plate 19 in its original and proper position. The outer head 22 serves as a locking element extending outwardly from the receptacle.

In operation, the prongs 10 may be inserted within the receptacle 23 in the usual standard manner. The shell 13 is rotated to a position so that the portion of the head 22 of the double head screw 20 will move inwardly through the slots 17 and align itself with the slot 18. By giving a simple twist to the shell 13 thereafter that portion of the double head screw 22 will move into the slot 18 as shown in Fig. 4 and thereafter rigidly secure the plug from being accidentally partially and unintentionally a. standardsreceptacle and without requiring installation of. an additional and special type receptacle. Within the United States all receptacles manufactured have been standardized in size and all have a screw for maintaining the wall plate 19 which may be replaced by the. double head screw 20 to be in proper position for thelociting action described above.

When one of our safety plugs has been, locked in proper position as described above there is no danger of the same being partially withdrawn with consequently large fluctuations in the current passing to the electrical d-e vice with which it is connected. This eliminates .ny danger of the device being burned out as a result of such fluctuations.

It. should be noted that our device efiectively prevents infants and small children. from removing the male. plug and thereby gaining access to the interior of the receptacle with consequent extreme danger. it should also be noted that the locking elements (17 and 18) of the plug is inherent within the plug itself.

The biggest advantage of our invention, of course, lies in the fact that the safety plug may be used with the standard receptacle currently sold within the United States. The sole alteration required is the inconsequential removal of a single screw and the substitution therefor of the double headed screw 20. Even. with a double receptacle installation, such as is shown in Fig. 1 only a single double headed screw need be substituted. In such an arrangement the same double headed screw 20 will cooperate with two separate shells l3 simultaneously to lock each of the shells securely in proper position.

It will, of course, be understood that various. changes may be made in the form, details, arrangement and pro portions of the various parts without departing from the scope of our invention.

What we claim is:

In an electric safety plug assembly for use in conjunction with a receptacle, the combinationof a double headed screw having an inner head and an outer head spaced outwardly therefrom, said heads being greater in diameter than the rest of the screw, said screw replacing the screw used ordinarily to secure the receptacle to the wall and performing its function of retaining the receptacle in place, the inner head of said screw abutting against the receptacle and the outer head being disposed out- 4. wardly therefrom; and a. safety plug engaged by said screw and secured to the receptacle thereby and comprising a main body of insulating material having a face adapted to register with the face of the receptacle, a pair of spaced electrical conductor prongs extending outwardly from said face of said body and adapted for ready insertion into said receptacle, said body being adapted to permit the electrical connection of said prongs with the wires of an electric cord, and a shell encircling said body and being unl-imitedly rotatable in either direction about said body and having a peripheral edge portion disposed adjacent to but inwardly of said face of said body, said shell being connected to said body to prevent. appreciable longitudinal movement with respect to. said body, the pe ripheral edge portion of said shell having a longitudinally extending slo-t extending inwardly from the outer edge thereof and having a circumferentially arranged slot communicating with said longitudinally arranged slot and disposed inwardly from the outer edge. of said peripheral portion, said longitudinal slot, receiving said outer head of said screw therein when said prongs are inserted into the receptacle. and said. circumferentially arranged. slot icceivingsaid outer head of said screw therein and positively and cooperatively locking said shell directly to said screw and the receptacle when said shell is rotated about.

its longitudinal axis after said prongs have been inserted into the receptacle and said outer head has been received within said longitudinally extending slot whereby accidental disconncction of said safety plug will be prevented.

References Cited in the file of this patent UNITED STATES PATENTS 1,520,421 Kolar Dec. 23, 1924 1,635,507 Seasock July 12, 1927 2,005,475 Schmidt June 18, 1935 2,046,221 Thomas June 30, 1936 2,199,560 Falter May 7, 1940 2,240,050 Nuebling Apr. 29, 1941 2,266,560 Mansfield Dec. 16, L941 2,569,037 Dalton Sept. 25, 195i FOREIGN PATENTS 721,168 Germany .1 May 28, 1942 

